The terms "acrylic acid," "acrylate,38 and "acryloyloxy" are used hereinafter in this specification to mean both acrylic acid and methacrylic acid, acrylate and methacrylate, and acryloyloxy and methacryloyloxy, respectively.
Recently, a method of curing coated films or printed images by irradiating with an electron beam has been put into practical use. Paints and inks which can be cured by this method are made from polymerizable compounds composed mainly of radical-polymerizable compounds which are prepared by appropriately combining prepolymers, oligomers, and monomers, all containing therein polymerizable unsaturated groups.
In comparison with more conventional heat-curing or ultraviolet ray-curing method, the cure time required in the electron beam-curing method, particularly with respect to productivity and resin characteristics, is usually within a second, so that the coated film layer or printed ink layer is subject to abrupt shrinkage in the course of curing thereof. As a result, the residual strain of the layer is severely increased because it is cured in a very short period of time, and thus the adhesive properties of paints or inks to substrates tend to be deteriorated in electron beam-curing method.
This tendency is particularly significant when a steel sheet is used as a substrate, unlike a paper or plastic substrate wherein the absorption phenomenon of paint or resin into the substrate and the graft reaction can be expected.
In order to eliminate the foregoing defects, various methods have been proposed, but no satisfactory methods have been discovered.
These methods can be divided into a group in which the resin structure and composition are intended to be improved and a group in which the irradiation process is intended to be improved.
Examples of methods falling in the former group include a method wherein compounds capable of reacting with a metal surface (e.g., a silane coupling agent, a titanate coupling agent, an epoxidized compound, and a polyisocyanate compound) are incorporated, a method wherein a phosphoric acid ester group and a carboxy group are compounded or introduced into the molecular skeleton of the resin, a method wherein a surfactant is incorporated, and a method wherein in order to reduce the shrinkage in the course of curing, a thermoplastic polymer is added.
Examples of methods falling in the latter group include a method wherein painting or printing is performed while heating the substrate, a method wherein after the irradiation, heating is applied to release the strain, and a method wherein multi-stage irradiation is applied to release the strain.
A large number of acryloyloxy-terminated polyesters wherein the polybasic acid unit and the polyhydric alcohol unit constituting the polyester portion are varied are known in the art. For example, polyester acrylates having an aliphatic polyester skeleton are described in U.S. Pat. Nos. 3,455,801, 3,455,802, 3,485,732 and 3,485,733, and polyester acrylates containing an aromatic dibasic acid unit are described in Japanese patent application (OPI) Nos. 1322/71, 883/73 and 134745/74 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") and Japanese patent publication Nos. 22164/73 and 8264/73.
Japanese patent publication No. 22164/73 and Japanese patent application (OPI) No. 134745/74 disclose terephthalic acid, isophthalic acid and a dicarboxylic acid having 4 or more carbon atoms as a starting material for production of polyester acrylates, but do not describe polyester acrylates as used in this invention, and include no disclosure of using the polyester acrylates on steel sheet.
The use of terephthalic acid, isophthalic acid and a saturated dicarboxylic acid having 4 or more carbon atoms as a starting material for polyester acrylates are also disclosed in Japanese patent publication Nos. 36956/73, 22995/77, 25438/77, 30431/79 and 30035/76 and Japanese patent application (OPI) Nos. 49027/78, 139692/78, 147024/78 and 65224/80.
Additionally, crystalline acryloyloxy-terminated polyesters having a polyester portion made from terephthalic acid and ethylene glycol are known. These crystalline acryloyloxy-terminated polyesters, however, cannot be used as materials for liquid paints or inks, since they have high melting points and are almost insoluble in generally used monomers, oligomers or solvents.
Furthermore, a number of polymerizable acrylates, such as acryloyloxy-terminated polyurethane (urethane acrylate) and acryloyloxy-terminated epoxy ester (epoxy acrylate), are known. It is also known that such polymerizable acrylates can be used as paints or inks which are to be cured by irradiation with ultraviolet rays or electron beams, or by heating. However, paints and inks prepared using such polymerizable acrylates are not sufficient in the adhesion to a steel sheet, such as a zinc electrogalvanized steel sheet, a zinc galvanized steel sheet, and a cold-rolled steel sheet, a chemically-treated steel sheet, a tin plate, a tin-free steel (TFS) sheet, etc. Furthermore, their performance is insufficient in applications where forming is performed after the painting or printing. Thus, these paints and inks are subject to limitations in thier applications.